The present disclosure relates to a fixing device for fixing a toner image to a recording medium and an image forming apparatus including the fixing device.
An image forming apparatus of an electrographic manner, such as a copying machine or a printer, includes a fixing device for fixing a toner image to a recording medium, such as a sheet.
For example, there is known a fixing device in which a fixing roller (a heating body) coming into direct contact with a pressuring roller (a pressuring body) to thereby form a fixing nip is configured as a heating member heated by an electromagnetic induction heating means. In this fixing device, the fixing roller is provided with a heating layer heated by the electromagnetic induction heating means, and then, the fixing roller is directly heated and a thermopile (an infrared ray detecting element) is provided integrally with a coil guide. Incidentally, in the coil guide, a cylindrical part in which a through hole for mounting has been formed is provided, and then, the thermopile is fitted into the hole of the cylindrical part and the hole is closed.
In the fixing device in which the pressuring body and the heating body come into direct contact with each other to thereby form the fixing nip, a first frame supporting the heating body may be approached to or separated from a second frame supporting the pressuring body, and thereby, pressurization or depressurization between the pressuring body and the heating body can be switched.
In the fixing device configured so that the thermopile (the infrared ray detecting element) is fitted into the cylindrical part, in a case where a mounting position of the thermopile is even slightly displaced, a path leading up to the thermopile of the infrared ray generated from the heating body may be interrupted by an inside wall of the cylindrical part, that is, the inside wall may conceal a detection range (a field of view) of the thermopile.
In addition, in order to reduce an apparatus construction for the sake of downsizing and space saving of the fixing device, it is necessary to install the thermopile at an upper part of the heating body as mentioned above. However, the thermopile installed at the upper part is prone to be influenced by a convection heat from the heating body, and accordingly, there is a possibility of mistaken detection of temperature. In particular, in the fixing device having the above-mentioned configuration, because the thermopile closes the hole of the cylindrical part, the convection heat (or a radiant heat) generated from the heating body stays in the cylindrical part, and accordingly, there is a possibility that the temperature detected by the thermopile is shifted from an actual temperature.
Further, as mentioned above, in the fixing device configured so that the first frame supporting the heating body is approached to or separated from the second frame supporting the pressuring body, if the thermopile is fixed to the movable first frame, there is a possibility that a failure, such as disconnection, of wiring of the thermopile occurs. Furthermore, if the coil guide into which the thermopile is fitted is provided independently from the movable first frame, because the heating body is moved together with the first frame, the detection position by the thermopile is shifted, and accordingly, there is a possibility that the heat of the heating body cannot be appropriately detected.
Incidentally, in order to prevent the convection heat of the heating body from having an influence on the thermopile, there is a case where the first frame may be constructed as a heat interrupting member between the heating body and the thermopile. In this case, an aperture for causing the infrared ray radiated from the heating body to pass therethrough is provided in the first frame and the thermopile is arranged to face to the heating body across the aperture. However, because the first frame is moved in order to switch depressurization or depressurization between the pressuring body and the heating body, the position of the aperture is shifted from the thermopile, and accordingly, there is a possibility that the thermopile cannot appropriately detect the heat of the heating body.
Thus, if the heat of the heating body cannot be appropriately detected, there is a possibility that high temperature of the heating body cannot be detected and the heating body is overheated, and moreover, there is also a possibility of firing.